[Proj] How grids work in PROJ4
warmerdam at pobox.com
Tue Jan 10 19:34:30 EST 2012
On Tue, Jan 10, 2012 at 3:49 PM, Michal Seidl <michal.seidl at gmail.com> wrote:
> I moved forward on implementing quite new Czech projection called "Krovak
> Modified" which is used in new ProjectedCRS EPSG:5224. I also want to
> implement vertical datum "Bpv" by use of +geoidgrids but I found myself
> lacking some basic information which I can not easily find on the web.
> 1) What kind of coordinate transformation is supported by PROJ 4.8.0?
> - towgs84?
> - nadgrids?
> - geoidgrids?
> - others?
Yes, the above, and obviously projected transformations (ie proj=tmerc).
> 2) Can I combined these transformation? And how they are applied? Look at
> these situations:
> cs2cs ... +proj=latlong +nadgrids=... +geoidgrids=... +towgs84=... +to
> +towgs84 ... +geoidgrids=... +nadgrids=... +proj=latlong
Only one horizontal datum transformation can be associated with a
coordinate system. So you should not currently mix +towgs84 and
> What happens?
> I mean by this:
> a) if there is +nadgrids and +towgs84 on the LEFT SIDE PROJ firstly changes
> LL by dL,dL from grid before 3D transformation
Only one or the other is going to work properly.
> b) if there is +geoidgrids on the LEFT SIDE PROJ thinks there is orthometric
> height on input so firstly it changes H by dh from grid to get ellipsoid
> height before 3D transformation
> c) on the RIGHT SITE it should work in a opposite order. Firstly do 3D
> transformation and than apply +nadgrids and +geoidgrids as there is
> ellipsoid height on the imput from 3D transformation
Right, with the caveat of only nadgrids or towgs84.
> I think PROJ should do this but I am not sure about it. I tried to do some
> test but I got confused. Is there some document how it exactly works?
I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Software Developer
More information about the Proj